You could implement it as either a subaccount per customer (in which case the balance is just the sum of all txns in that account) or have 'apartment' as another dimension (field) you store (optionally) for each transaction.
Either way you can get a sum of all txns that have hit that apartment's AR account.
Like a sub-account? This is probably the mindfuck part. So every apartment owner who opens up their statement is seeing a filtered balance?